The present invention concerns a method of detecting when a tire reaches one or more predetermined wear thresholds, which may be detected when the tire is stopped or while running. The invention also concerns a detection unit intended to be incorporated in such a tire for implementing this method, a tread pattern element for such a tire comprising this unit, a tire tread provided with this unit, and a tire comprising this tread. The invention also comprises a system of implementing this method comprising this unit, a tire/wheel assembly for a motor vehicle and such a vehicle comprising this system.
It is known how to provide wear detectors for tread patterns on tires for motor vehicles.
The German patent document DE-A-197 45 734 concerns a wear detector for a tire which forms part of the tire (see FIGS. 2 and 3 of this document). More precisely, in the mass of a tread pattern strip on the tire, metallic wires are provided, forming electrically conductive loops which extend at different heights from the top of the bar and which are connected to a detection circuit contained in the tread on the tire, radially below the tread strip.
When a vehicle equipped with this tire is running, these loops are broken successively because of the wear on the strip, so that they form open switches, and the detection circuit delivers a signal representing these breaks to an assessment unit present in the vehicle.
One major drawback of the wear detector described in this document is that the detection circuit can comprise electrically active components in the mass of the tire.
One aim of the present invention is to propose a method of detecting one or more predetermined wear thresholds on a tire comprising tread pattern elements, which remedies this drawback.
To this end, the detection method according to the invention comprises the assigning a discrete wear variable representing the wear on the element to a discrete impedance variable representing the impedance of a passive tuned circuit contained in at least one of the tread pattern elements.
According to another characteristic of the invention, the method comprises the measurement of resonant frequencies of the tuned circuit which represent, respectively, values of the impedance variable and a comparison of each of these measured frequencies with one or more reference resonant frequencies. The reference resonant frequencies each represent a wear state on the element which does not reach at least one of the thresholds.
According to another characteristic of the invention, the tuned circuit comprises a coil and at least a quantity n (n≧1) capacitor(s) connected in parallel to the terminals of the coil, the impedance variable being a function of the inductance of the coil or of the capacitance of the capacitor or capacitors, the inductance (the first case) or the capacitance (the second case) being designed to vary discretely when the wear threshold or thresholds are reached.
In a first embodiment, this method is such that a discrete increase in the inductance represents the tire reaching a wear threshold or thresholds, measured by a discrete reduction in the resonant frequency.
According to another aspect of the first embodiment, and considering that the tread pattern elements are intended to be in contact with the ground through their respective external faces, the method provides the coil with one or more short-circuit conductive lines each comprising a vertex which is contained inside a tread pattern element and which is directed towards the external face of the tread pattern element, the respective vertices being situated at different heights in the radial direction of the tire, a wear threshold being reached when the external face of a tread pattern element has broken the vertex of a conductive line.
According to a second embodiment, the method includes the step of determining that a discrete reduction in the capacitance has occurred, the discrete reduction representing the tire reaching a threshold or one of the thresholds, this reaching then being measured by a discrete increase in the resonant frequency.
Further to the second embodiment, and considering again that the tread pattern elements are intended to be in contact with the ground through their respective external faces, the method comprises the step of determining the discrete reduction in capacitance measured in at least two capacitors, each provided with at least one conductive branch, wherein two or more branches are arranged in parallel, the at least one branch comprising a vertex which is contained inside the tread pattern element and which is directed towards the external face of the tread pattern element, two adjacent capacitors being connected together by at least one branch, the vertices respectively relating to pairs of adjacent capacitors being situated at different heights in the radial direction of the tire, and the wear threshold or thresholds being reached when the external face has broken the vertex of the branch or one of the branches.
Advantageously, the impedance variable is measured over all or part of the width of the tire (i.e. the axial dimension thereof).
It should be noted that this method according to the invention makes it possible to indirectly detect the breaking of the short-circuit line or lines or parallel branches by measuring resonant frequencies, providing only electrically passive components in the tire.
In the present description, “vertex” means the portion of the short-circuit line or lines or parallel branch which is closest to the external face of the corresponding tread pattern element, in the radial direction of the tire.
According to one embodiment of the invention, the or each short-circuit line or parallel branch forms substantially an inverted U, whose connecting web (which may be straight or curved for example) forms the aforementioned vertex and is contained inside one of the tread pattern elements so that the web or its tangent are substantially parallel to the external face of the element.
According to a variant embodiment of the invention, the at least one short-circuit line or parallel branch forms substantially an inverted V, whose vertex is consequently practically at one point.
Another aim of the invention is to propose a detection unit intended to be incorporated in a tire for implementing the method according to the invention for detecting the reaching of one or more predetermined wear thresholds.
To this end, a tire casing according to the invention contains a passive electrical circuit which is covered with a rubber composition which can be used in the tread of the tire and which comprises a coil printed on a substrate and a quantity n (n≧1) capacitors connected in parallel to the terminals of the coil, and in that:
in a first embodiment, the coil is provided with one or more short-circuit lines each comprising a vertex which is contained inside the unit, the vertices being situated at different heights on the coil, or
in a second embodiment, each capacitor is provided with one or more branches in parallel, each comprising a vertex which is contained inside the casing, two adjacent capacitors being connected together by one or more branches, the vertices respectively relating to adjacent pairs of capacitors situated at different heights on the capacitors.
According to an example embodiment of the invention relating to the first case, the coil is provided with a single short-circuit line connecting a terminal of the coil to a turn thereof.
According to another example embodiment of the invention relating to the first case, the coil is provided with a quantity m (m≧2) short-circuit lines connecting the same terminal of the coil to a same quantity m turns thereof, the respective heights of the line vertices as from the coil decreasing in the direction of the turns approaching the terminal.
According to an example embodiment of the invention relating to the second case, the passive circuit comprises two capacitors which are connected in parallel to the terminals of the coil and which are connected together by the branch in parallel or by a pair of such branches, the respective vertex or vertices of which occupy one and the same height from the capacitance, for detecting a single wear threshold on the tread pattern element containing the vertex or vertices.
According to another example embodiment of the invention relating to the second case, the passive circuit comprises at least three capacitors which are connected in parallel to the terminals of the coil and which are connected in pairs to each other by a branch in parallel or by a pair of such branches, so that the circuit comprises at least two branches whose respective vertices occupy different heights from the capacitors for detecting at least one intermediate wear threshold on the tread pattern element containing the vertices.
Another aim of the invention is to propose a tread pattern element for a tire intended to be attached to the tread of the tire, for implementing the aforementioned method according to the invention.
To this end, a tread pattern element according to the invention comprises a detection unit as defined previously, the tread pattern element comprising a base intended to be fixed to the tread, an external face intended to be in contact with the ground and the passive electrical circuit between the base and the external face, the vertex of the at least one short-circuit line or branch in parallel being adjacent to the external face and being situated so as to be recessed from the external face.
“Tread pattern element” means in the present description any element in relief which is intended to be in contact at one time or another with the ground following its incorporation in the tread, i.e. at the very start of running, or after wear on this element has begun. This element can thus consist of a block, for example, substantially parallelepiped or cylindrical in shape, or a circumferential rib with a variable transverse section (i.e. extends over all or part of the circumference of the tread).
It should be noted that the tread pattern element according to the invention can advantageously consist of a “wear indicator”, for example, in the form of a block or rib with a height substantially less than that of the tread pattern elements which are intended to be in contact with the ground from the start of running.
Another aim of the invention is to propose a tread for a tire and such a tire, for implementing the method according to the invention for detecting the reaching of one or more wear thresholds.
According to a first embodiment of the invention, a tread according to the invention is such that it comprises at least one tread pattern element such as the aforementioned, and a tire according to the invention is such that it comprises the tread.
It should be noted that a tire according to the invention might not include the unit according to the invention but only the passive electrical circuit according to the invention, which would then be without the rubber composition which coats this electrical circuit in order to form the unit.
Advantageously, the tread according to the invention also comprises a conductive short-circuit loop which is substantially parallel to the external face of the tread and which connects together the terminals of the tuned circuit for measuring the wear on all or part of the width of the tread when the loop is cut by the external face because of the wear.
According to a second embodiment of the invention, a tire for implementing the wear detection method comprises a carcass reinforcement which is radially surmounted by a crown reinforcement and which is extended laterally by side walls, a tread comprising tread pattern elements radially surmounting the crown reinforcement. This tire comprises the detection unit according to the invention, this unit being substantially in the shape of a tack whose head is connected to a radially internal face of the tire facing one of the tread pattern elements and whose stem contains said short-circuit line or lines or branch or branches in parallel and extends radially through the tire from the radially internal face and as far as the inside of the tread pattern element.
It should be noted that the tires according to these first or second modes comprise only electrically passive components in their mass.
Another aim of the invention is to propose a tire/wheel assembly for a motor vehicle comprising a wheel and a tire mounted on the wheel, for implementing the aforementioned wear detection method according to the invention.
To this end, the tire/wheel assembly according to the invention is such that the tire corresponds to the first or second aspects according to the invention, and that the passive electrical circuit in the tire is coupled to an active interrogation circuit which is fixedly mounted to the wheel and which comprises a frequency-sweep energy generator and means for detecting the tuning frequencies between the passive circuit and the interrogation circuit.
According to a preferred embodiment of the invention, the interrogation circuit comprises, in addition to the generator, a capacitor, a resistance and a coil coupled to the coil of the passive circuit, and the tuning frequency detection means is then connected to the terminals of the resistance to measure the voltage at these terminals.
According to an example embodiment of the invention, the interrogation circuit is mounted on an inflation valve of the wheel (i.e. an instrumented valve).
According to a variant embodiment of the invention, the interrogation circuit is mounted on a module for measuring the internal pressure of the tire mounted on the wheel.
According to another variant embodiment of the invention, the interrogation circuit is mounted directly on the wheel rim.
According to another characteristic of the invention, the interrogation circuit is provided with transmission means intended to transmit tuning frequency information to a central unit which is portable or mounted inside the vehicle, which central unit is intended to inform a user that at least one predetermined wear threshold of the tire has been reached.
Another aim of the invention is to propose a motor vehicle comprising a chassis, tire/wheel assemblies and a suspension device providing a flexible connection between the chassis and the tire/wheel assemblies, for implementing the aforementioned wear detection method according to the invention.
To this end, the vehicle according to the invention is such that it is equipped with at least one tire according to the first or second aspects according to the invention, and comprises on the chassis or on the suspension device an active interrogation circuit to which there is coupled the passive electrical circuit contained in the tire, the interrogation circuit comprising a frequency-sweep energy generator and a means of detecting the tuning frequencies between the passive circuit and the interrogation circuit.
According to another preferential characteristic of this vehicle, this interrogation circuit also comprises a capacitor, a resistance and a coil coupled to the coil of the passive circuit, and this tuning frequency detection means is connected to the terminals of this resistance in order to measure the voltage at these terminals.
According to one example embodiment of the invention, the interrogation circuit is mounted on a wall of the chassis corresponding to the wheel passage area.
According to a variant embodiment of the invention, the interrogation circuit is mounted on the suspension device, for example, on a suspension arm or on a damper cup forming a stop, on which a damper spring of the suspension arm bears.
According to another characteristic of this vehicle according to the invention, the interrogation circuit is provided with transmission means intended to transmit tuning frequency information to a central unit which is portable or mounted inside the vehicle, which central unit is intended to inform a user that a predetermined wear threshold of the tire has been reached.
Another aim of the present invention is to propose a detection system for implementing the method according to the invention for detecting that one or more predetermined wear thresholds of a tire have been reached.
This system comprises the detection unit according to the invention and an active interrogation circuit to which the passive circuit of the unit is coupled, this interrogation circuit comprising a frequency-sweep energy generator and a means of detecting the tuning frequencies between the passive circuit and the interrogation circuit.
The interrogation circuit preferably comprises, in addition to this generator, a capacitor, a resistance and a coil coupled to the coil of the passive circuit, and this tuning frequency detection means is then connected to the terminals of the resistance in order to measure the voltage at these terminals.
Also preferentially, this interrogation circuit is provided with transmission means intended to transmit tuning frequency information to a central unit, which may be portable or mounted inside a vehicle and intended to inform a user that a predetermined wear threshold of the tire has been reached.